Various systems have been constructed which track the location of at least one moveable object in a volume and frequently also track the spatial orientation of the object. The object generally is rigid and is sometimes referred to as a rigid body. Sometimes the moveable but rigid parts of a body are tracked. Tracking an object simply means measuring the 3-d location and/or orientation of the object at least once—but more typically measured frequently and at known instants over a period of time. In some cases, the object is a tool, medical instrument, probe, or pointer, for example. Systems which employ optical means of volumetric tracking typically simplify the task by measuring the location of one or more easily detectable, artificial markers affixed to the object. One kind of typical marker is a energized, point-like light source such as an infrared light emitting diode (LED). This is exemplified by the 3D Creator™ system manufactured by Boulder Innovation Group (Boulder, Colo., USA). Another kind of typical marker is a retro-reflective spot or ball illuminated by a light source typically near each of two or more video cameras, which each stereographically track the centroid of images of each retro-reflective marker. This is exemplified by the Polaris® system made by Northern Digital, Inc. (Waterloo, Ontario, Canada). See FIG. 1a, which is a drawing of an exemplary prior-art system.
An alternative, passive marker is a small high-contrast marker illuminated by ambient lighting, where the marker includes at least one identifiable reference feature point within a recognizable pattern. An example marker is a pattern of alternating adjacent black and white pie-shaped sectors converging on a central saddle point, as described in U.S. Pat. No. 6,978,167. FIG. 1b illustrates an example of such a prior-art marker. Such markers may be tracked stereographically using a system of at least two video cameras, such as the MicronTracker system sold by Claron Technology, Inc. (Toronto, Ontario, Canada).
Whether the marker is an LED, a small retro-reflective spot, a crosshair, or the central vertex of a high contrast pattern, these systems essentially locate or track an individual reference point on the marker. Furthermore, the reference point typically is projected optically onto only a very small region—that is, a few pixels—of the array of image pixels of a video camera or other optical imager. FIG. 1c is an example of an actual image of a retro-reflective ball occupying only a few pixels from a pixel array in a video camera.
To fully track the 3-d orientation of an object therefore requires that at least three such reference point oriented markers be affixed some distance apart from each other in a non-collinear arrangement on the object to be tracked. The space between such prior-art markers remains unused for measurement purposes. Further, distinguishing between two or more objects which each have their own similarly arranged markers may be challenging technically. A common approach is to arrange the markers affixed to one object in a significantly different geometrical pattern from the markers affixed to another object being tracked. For example, the number of markers on one object and/or the distances between the markers may differ from the pattern of another object. An example is the retro-reflective markers on each of the various medical instruments tracked by the VectorVision system of BrainLAB AG (Feldkirchen, Germany). FIG. 1d is a photograph of a typical instrument—a probe pointer in this case—with four non-collinear retro-reflective balls as the markers.
Some prior-art systems which employ actively illuminated markers (such as LEDs or retro-reflective balls) use a scheme of background subtraction. Therein a video frame is acquired and saved at some time when the markers are not illuminated. A video frame herein means the 2-d set of pixel amplitudes output from the pixel array of a camera, which is sometimes also called a raster. That saved video frame is then subtracted—pixel-by-pixel—from a second video frame acquired when the markers are illuminated. The difference between the two frames typically leaves only significant images for the markers, which are then easier to locate and process. Existing known techniques, like background subtraction, may be applied to the present invention but may not otherwise be specifically described herein.
More sophisticated video systems attempt to recognize and track whole objects or complex geometric shapes. This may require complex templates of the object and require correlating a 3-d rotated view of the template with the image of the object. Such systems may require substantial computational resources to perform pattern matching, may be too slow for some real-time applications, or may not satisfy spatial accuracy requirements. An example of such a system was the Eagle Eye® system of Kinetic Sciences Inc. (Vancouver, BC, Canada). Measuring the location or orientation of objects which have no well-defined or artificial markers—such as in tracking vehicles or faces—may be even less reliable and less accurate. Further, it may be nearly impossible to unambiguously determine the orientation of symmetrical or amorphous objects which possess no natural high contrast details or unusual geometrical features to implicitly serve as the equivalent of markers.